Tunable optical add/drop multiplexers (TOADM) or reconfigurable optical add/drop multiplexers (ROADM) are an integral part of next generation optical networks, and they are the building blocks of Agile Optical Networks, because they provide the flexibility to remotely tune or reconfigure any or all wavelengths, thereby reducing time-to-service, simplifying the network, and streamlining planning and management.
The basic requirements for TOADM's or ROADM's are: flat-top filter characteristic for drop/add function, low insertion loss in express channels, no-hit to express channels during switching process, low cost, and reduced form factor.
Conventional add/drop multiplexers, such as those disclosed in U.S. Pat. Nos. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al, 6,707,959 issued Mar. 16, 2004 to Ducellier et al, 6,810,169 issued Oct. 26, 2004 to Bouevitch et al and 7,027,684 issued Apr. 22, 2006 to Ducellier et al, comprise wavelength dispersing elements, e.g. diffraction gratings, for dispersing an optical signal into constituent wavelength channels, and an array of micro-electro-mechanical mirrors for independently directing each of the wavelength channels to selected output ports.
Other conventional TOADM's, which are based on thin film filter (TFF) technology, such as those disclosed in U.S. Pat. Nos. 6,728,041 issued Apr. 27, 2004 to Chang et al; and 6,847,757 issued Jan. 25, 2005 to Chang et al, rely on rotating the thin film filter to select which wavelength channel is separated from the input optical signal.
The key issue of the technology for rotating tunable filters and TOADMs is how to control the angle and the position of the reflected and transmitted optical beams, when they pass through or are reflected by the TFF. Conventional rotating tunable optical filters, such as the one disclosed in U.S. Pat. No. 5,361,155 issued Nov. 1, 1994 to Chiaroni et al, uses a co-axial plate to compensate the walk-off introduced by the change in angle of the rotating TFF. U.S. Pat. No. 6,714,743 issued Mar. 30, 2004 to Hsieh et al discloses a tunable filter in which a mirror is rotated simultaneously with the TFF, enabling the angle of the reflected beam to be unchanged, i.e. enabling the reflected and transmitted beams to travel in parallel. Unfortunately, the mirror and the TFF need to be rotated simultaneously, necessitating a transmissive mounting block assembly supporting the mirror and the TFF, and a highly accurate alignment process.
In the instance where it is desired to change the added or dropped wavelength channel, it is easily accomplished without having to replace the filter element, or the entire add/drop device with another having the desired bandpass characteristics. This is typically accomplished by repositioning the filter with respect to an incident optical beam. However, one unfortunate aspect resulting from dynamically tuning an add/drop device is that intermediate wavelength channels, those channels having wavelengths existing between that of the initially tuned channel and that of the finally tuned channel, will each sequentially exhibit a temporary loss of signal continuity as the filter is tuned to each respective channel's wavelength. This occurs because the point of incidence of the optical beam upon the filter, in transitioning from a point corresponding to the initially tuned channel to a point corresponding to the finally tuned channel, passes filter locations corresponding to each of the intermediate channels. When the composite optical signal light strikes filter locations corresponding to intermediate wavelength channels, intermittent data loss from those intermediate wavelength channels results. Such an intermittent loss of data is often referred to as a so-called “hit.” The deleterious effects of a data hit, to the devices for which the data is intended to be transmitted, are well known. Devices exposed to such a data loss must either compensate for the loss of data, or request retransmission of the lost data. Ultimately, such data loss results in diminished quality of service, decreased bandwidth efficiency, or both
An object of the present invention is to overcome the shortcomings of the prior art by providing a TOADM using a tiltable mirror device and a fixed thin film filter. The present invention further comprises optical retro-reflectors working with the tiltable mirror, keeping both direction and position of a reflected beam fixed, while tilting the mirror, ensuring low coupling loss in all ports for add/drop and express channels.